Some of the blood vessels in the lung expand when the lung is inflated with air while the intravascular pressure is held constant relative to atmospheric pressure. These vessels (i.e., the extra-alveolar vessels and the alveolar corner vessels), together with the pressures (total and fluid) in their surrounding interstitium, affect segmental pulmonary vascular resistance and compliance, the formation and clearance of pulmonary edema, the distribution of perfusion and, consequently, gas exchange in both normal and edematous lungs. Our previous studies have led us to a new model of the pulmonary microcirculation that is critically dependent on the pressures in the interstitial spaces located in alveolar corner regions. In addition, we have recent observations suggesting that the effects of lung inflation on the total pressure surrounding arterial extra-alveolar vessels vary with vessel size. The purpose of this proposal is to investigate certain structural and functional aspects of the extra- alveolar vessels, the alveolar corner vessels, and their surrounding interstitium, with the idea of determining (1) if lung interdependence might affect the small versus larger extra-alveolar vessels differently; (2) whether the alveolar corner vessel anatomy and physiology that we have observed just under the pleural surface is representative of that located deeper in the lung parenchyma, and (3) how alveolar surface tension, lung volume and transpulmonary pressures affect the interstitial pressures in alveolar corner regions.